(1) Field of the Invention
The present invention generally relates to a vehicle speed control apparatus, and more particularly to an apparatus for controlling a vehicle speed in response to a change of a road holding ability relating to tires of an automotive vehicle during running operation so as to prevent the vehicle speed from becoming excessively high when the road holding ability is reduced.
(2) Description of the Related Art
In known automotive vehicles, the vehicle running performance is influenced by not only the vehicle structure but also the tire condition.
Japanese Laid-Open Patent Application No. 62-59169 discloses a proposed steering performance control apparatus which maintains steering performance in a certain condition when the tire inflation pressure changes. In the proposed apparatus in the above mentioned publication, the steering performance is adjusted so as to reduce the understeer effect of the vehicle when the tire condition is changed to a condition in which the front tire inflation pressure is lower than the rear tire inflation pressure. On the other hand, when the tire condition is changed to a condition in which the front tire inflation pressure is higher than the rear tire inflation pressure, the steering performance is adjusted so as to increase the understeer effect of the vehicle.
Japanese Laid-Open Patent Application No. 60-148771 discloses a proposed steering control apparatus. In this proposed apparatus, the turning angle relating to the rear wheels of a four-wheel-steering vehicle is controlled in response to a tire inflation pressure change.
However, the tire inflation pressure is not the only factor that influences the vehicle running performance. The vehicle running performance is influenced when the tire wear is increased, or when the spare tires which are different from standard tires are mounted on the vehicle. For example, there is a case in which a "tempa" spare tire is mounted on the vehicle. The "tempa" spare tires are one of the known spare tires, and the dimensions of the "tempa" spare tire (such as the diameter and the breadth) are smaller than the dimensions of the standard tire.
Generally, the decrease in the tire inflation pressure, the increase in the tire wear, and the mounting of the spare tires will lower the road holding ability of the tires. When the vehicle is subjected to one of these factors during the running operation, the vehicle stability and controllability performance becomes worse and the braking distance is increased.
FIG. 1A shows the relationship between the vehicle speed and the vehicle running stability and controllability performance. As shown in FIG. 1A, if the vehicle speed is increased to a higher speed, the running stability and controllability performance becomes worse. In FIG. 1A, a characteristic curve of the vehicle running stability and controllability performance when the tire condition is normal is indicated by a dotted line, and a characteristic curve of the vehicle running stability and controllability performance when the tire condition is abnormal is indicated by a solid line.
FIG. 1B shows the relationship between the vehicle speed and the braking distance. As shown in FIG. 1B, if the vehicle speed becomes higher, the braking distance is increased. In FIG. 1B, a characteristic curve of the braking distance when the tire condition is normal is indicated by a dotted line, and a characteristic curve of the braking distance when the tire condition is abnormal is indicated by a solid line.
In the conventional devices described above, the steering performance of the vehicle is prevented from being lowered when a tire inflation pressure reduction is detected. However the measures taken by the above described conventional devices are insufficient to increase the vehicle running performance and safety. With the conventional devices, it is difficult to prevent the vehicle running stability and controllability performance or the braking performance from being lowered when the road holding ability of the tires is reduced.